1. Field of the Invention
The present invention relates to an information terminal and a battery remaining capacity calculating method.
2. Related Background Art
Examples of a small-sized information terminal using a battery are a digital camera, a portable moving image reproducing apparatus, a portable sill image reproducing apparatus, and so on. Recently, a unit such as a hard disk drive which exerts a large influence on load change is mounted in such a small-sized information terminal, and hence the load on the battery changes greatly according to the driving state of the unit.
It is very important to users to precisely display a battery remaining capacity as means for guessing how much more time the information terminal can be used. Methods of detecting a battery remaining capacity are a method of measuring charging/discharging currents (See Japanese Patent Application Laid-open No. Hei 5-66251, Japanese Patent Application Laid-open No. Hei 7-260839, for example) and a method of measuring a battery voltage (See Japanese Patent Application Laid-open No. Hei 6-51876, Japanese Patent Application Laid-open No. Hei 11-55372, for example).
However, the method of measuring the charging/discharging currents requires a current measurer, a storage to store data on a measured current, and a CPU to calculate a battery remaining capacity from the data, which hinders size reduction and cost reduction. Hence, it is not suitable for the small-sized information terminal.
On the other hand, in the method of measuring the battery voltage, because of a battery characteristic, the external voltage changes according to internal impedance when the load changes. Therefore, in the information terminal with a built-in unit having large load change, a precise battery remaining capacity cannot be detected. For example, the following situation occurs: When the information terminal is powered on, the load increases, and thereby the battery remaining capacity is displayed at a low value, and when the information terminal is stabilized thereafter, the load decreases, and thereby the battery remaining capacity increases. Such a change in the display of the battery remaining capacity is confusing and very inconvenient to the users.
A method of correcting the influence of load change is a method of measuring a current flowing through a load and correcting it as disclosed in Japanese Patent Application Laid-open No. Hei 5-66251. However, additional components such as a current detector and a comparison voltage corrector become necessary, which causes cost increase. Moreover, the information terminal, for which size reduction is indispensable, needs to avoid the addition of new components for the purpose of detecting the battery remaining capacity as much as possible. Further, the insertion of a resistance for detecting the current into a power supply line causes a drop in voltage and wasteful power consumption, and hence it is not suitable for the information terminal which requires the longest possible time driven by the battery.
Furthermore, the hard disk drive is accessed regularly at certain time intervals while the information terminal is reproducing moving images or sound or playing a slide show, and hence the load on the battery changes greatly at the certain time intervals. Namely, the load while the hard disk drive is being accessed and the load while the hard disk drive is not accessed are very different, and along with this, the load current also changes. Therefore, there is a problem that the battery remaining capacity is difficult to display precisely while the information terminal is reproducing and playing the slide show or the like.